The present invention relates to a structure and a manufacturing method of light emitting diode (LED), and more particularly relates to a LED structure having an electrode with a schottky contact and to a manufacturing method thereof. By reducing the quantity of carriers in an area under the electrode, the carriers can be distributed over the active region efficiently. Thus, the output intensity of light can be enhanced.
Nowadays, because LED has advantages of low manufacturing cost, low manufacturing difficulty level, easy and convenient installation and good development future, LED is used widely in daily life, such as electronic bulletin boards, indicator lights and car taillights, etc. However, how to further enhance the irraidation efficiency of LED is still a target on which engineers are working.
Referring to FIG. 1, FIG. 1 is a cross-sectional view showing the structure of a conventional LED. The conventional LED is grown by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). First, a buffer layer 102, a first confining layer 104, an active layer 106, a second confining layer 108 and a window layer 110 are formed on a substrate 100 sequentially, and then, by performing an evaporation step, a upper metal electrode 112 is formed on the window layer 110 and a backside electrode 114 is formed by evaporation beneath the substrate 100.
Current leaves or enters the window layer 110 through the upper metal electrode 112 of LED 90, and then the carriers arrive the active layer 106 after passing the second confining layer 108 uniformly by diffusion. Therefore, due to the carrier recombination, the active layer 106 ejects out photons thereby emitting light for the LED 90.
As the aforementioned description, the main current flow path is an area under the upper metal electrode 112, and the area contains plenty of carriers, so that the carriers can not be spread efficiently to the whole window layer 110 and to the whole chip. Hence, the main radiation combining area 116 is located on the center of the active layer 106 (the area beneath the upper metal electrode 112).
However, since most of carriers are only injected to the position under the upper metal electrode 112 during the operation of LED, and due to the Current Crowding Phenomenon that the carriers are concentrated under the upper metal electrode 112, there are no enough carriers to perform radiative recombination in the other position of the active layer 106. Thus, most of light emitted from the active layer 106 is either blocked and reflected by the upper metal electrode 112, or is absorbed by semiconductor, so that the emitting efficiency of LED 90 is decreased and is not be satisfied.
Referring to FIG. 2, FIG. 2 is a cross-sectional view showing the structure of another conventional LED. In order to resolve the aforementioned problem, the thickness of the window layer is increased for enhancing the spreading current. Another method of improving the Current Crowding Phenomenon is to formed a current blocking layer 118 beneath the upper metal electrode 112 in the subsequent process as shown in the FIG. 1 after forming the second confining layer 108 as shown in the FIG. 1. The carriers are blocked from moving downward by using the energy barrier and the electric field induced in the depletion region in the current blocking layer 118, so that the carriers beneath the upper metal electrode 112 are forced to be distributed allover the chip, and the main radiation combining area 116 is located on the active layer 106""s other areas which are not under the upper metal electrode 112. Therefore, the photons emitted from the active layer 106 will not be blocked by the opaque upper metal electrode 112, so that the light output intensity is enhanced.
Nevertheless, in the method of increasing the thickness of the window layer 110 for enhancing the spreading current, the thickness of the window layer 110 has to be about 5 xcexcm or over, which takes longer production time for forming the epitaxy and thus increases the production cost. In the other method of forming the current blocking layer 118 on the second confining layer 108, the current blocking layer has to be formed by using MOCVD twice. After the second confining layer 108 is formed, the chip needs to be moved out of the chamber for forming the current blocking layer 118, and after the current blocking layer 118 is formed, the chip is moved into the chamber again for performing the subsequent steps to complete the remaining structure. The additional step for forming the current blocking layer 118 results the extension of production time, the increase of production cost and the decrease of yield.
In view of the background of the invention described above, the LED with high efficient is urgently needed, so that many conventional methods for improving the light efficiency of LED are developed. In a conventional LED manufacturing process, for improving the current crowding phenomenon, a current blocking layer is formed on the second confining layer for blocking the carriers from moving downward and for forcing the carriers to spread around the chip. However, forming the current blocking layer has to employ MOCVD twice, before performing the subsequent steps. Because of additional step for forming the current blocking layer in the conventional manufacturing process of LED, the production time is extended, and the production cost is increased.
It is the principal object of the present invention to provide a LED structure and a manufacturing method thereof, and more particularly to provide a LED structure having an electrode with a schottky contact, and to provide a method for manufacturing the LED structure. In the present invention, by forming the schottky contact between the metal electrode and the window layer, the carriers are blocked from moving downward by using the energy barrier, and are forced to spread out, thereby enhancing the light output intensity.
In accordance with the aforementioned purposes of the present invention, the present invention provides a structure and a manufacturing method of LED, and the manufacturing method comprises: providing a first-type substrate; forming a first-type buffer layer on the first-type substrate; forming a first-type first confining layer on the first-type buffer layer; forming an active layer on the first-type first confining layer; forming a second-type confining layer on the active layer; forming a second-type window layer on the second-type confining layer; forming a first metal electrode beneath the first-type substrate, wherein there is a first ohmic contact surface between the first metal electrode and the first-type substrate; forming a second metal electrode on the second-type window layer, wherein there is a schottky contact surface between the second metal electrode and the second-type window layer; forming a third metal electrode on the second metal electrode, wherein the third metal electrode has a feature of high melting point; forming a fourth electrode on the third metal electrode and the second-type window layer, wherein there is a second ohmic contact surface between the fourth electrode and the second-type window layer; and forming a fifth metal electrode on the fourth electrode, wherein the fifth metal electrode has a feature of good adhesion, and when the first-type substrate, the first-type buffer layer and the first-type first confining layer are n-type, the second-type second confining layer and the second-type window layer are p-type, and when the first-type substrate, the first-type buffer layer and the first-type first confining layer are p-type, the second-type second confining layer and the second-type window layer are n-type.
Since there is a schottky contact surface between the second metal electrode and the second-type window layer, and the third metal electrode having the feature of high melting point is located between the second metal electrode and the fourth metal electrode, thus the carriers are blocked from moving downward and forced to spread out, and the schottky contact can be maintained, for enhancing the light output intensity by using the energy barrier. Thus, the thickness of the current-spreading layer can be diminished with a proper adjustment of the process, wherein the current blocking layer is no longer need, and the efficiency of carriers spreading around the chip can be maintained at the same time. Hence, the epitaxial material cost and the production time can be reduced because no additional current blocking layer is required, thereby increasing the throughput and the yield.